A wide variety of diagnostic techniques exist in the practice of medicine, including imaging modalities such as X-ray tomography, magnetic resonance imaging (MRI), and various nuclear-medicine imaging techniques. Molecular imaging techniques can use a tracer that is introduced to the body of a subject and selectively absorbed by parts of the body in which specific physiological processes are occurring, for example malignant tumor growth. An image of the body in such situations can selectively highlight both primary and secondary tumors, which can be extremely important in evaluating the progression of cancer.
In addition to imaging techniques, non-imaging diagnostic procedures also play an important role in medicine. Beyond the wide variety of blood tests that are available, for instance, other techniques may directly measure glomerular filtration rate (GFR), which can be a good measure of kidney function, where the standard are measurements of inulin clearance rate, a procedure that involves the infusion of inulin, and ongoing sampling of blood and urine to track the rate at which inulin is cleared from the blood. This test takes a considerable length of time to perform. For this reason, physicians may rely instead on indirect measurements of GFR based on serum creatinine levels coupled with data on body type. Among other possible needs, the inventors of the present disclosure have recognized that a GFR test that could be performed in real time could be of considerable value in identifying and managing acute kidney injury, or even the existence of chronic kidney problems that might contraindicate the use of contrast agents such as iodine.
With respect to existing imaging modalities, MRI can provide high detail, particularly of soft tissue. This technique may also be tailored so that contrast reflects morphology and also function and physiological processes. It has been recognized that longitudinal and transverse spin relaxation rates, 1/T1 and 1/T2, are different in tumor and healthy tissues. Differences in blood flow can also be translated into MRI contrast, a technique that has been central to functional MRI (fMRI) studies of the brain. MRI can have the drawback, however, that a relatively large number of nuclear spins is needed to get reasonable signal-to-noise (SNR).
In contrast to MRI, nuclear-medicine studies may utilize a variety of radioactive tracers that are explicitly introduced into the subject. These tracers can be chemically attached to various molecules that are selectively absorbed by the body, making it possible to probe specific processes or potential pathologies within the body. Detection can be accomplished through gamma-ray detection, and with the exception of positron emission tomography (PET) studies, imaging may rely on the use of gamma-ray cameras, which may be composed of an array of gamma detectors.
Because gamma-ray cameras use collimators, however, they may detect only a small number of the gamma rays incident upon them. Resolution is generally quite limited as a result.
In the mid 1990's, it was shown that certain noble gases, such as 129Xe and 3He, laser-polarized using the technique of spin-exchange optical pumping ([1]) could be used to image the gas space of lungs with unprecedented resolution ([2]). Xenon is lipophilic, and is known to dissolve into the blood stream; it is used both as anesthetic and as a contrast agent for CT of the brain. There has thus been interest in using laser-polarized 129Xe to probe parts of the body other than the lungs, but due to the limited amount of 129Xe that is delivered to distill parts of the body, and the small resulting signals, there has been only limited research in this area.
Despite the early work described in reference [3], the techniques of pulse NMR have not been employed while monitoring the nuclei using gamma detection. Furthermore, the question of whether pulse NMR techniques can be used to image radioactive isotopes while using spatial anisotropies in the emission of gamma rays as a means of detection has not been explored. This may be due to the fact that there are several important differences between detecting spatial anisotropies in gamma emission and the types of RF electromagnetic signals that are detected in magnetic resonance (MR) studies.
It is with respect to these and other considerations that the various embodiments described below are presented.